This invention relates to tunable optical filters utilizing electro-optic materials.
A number of uses has been developed for solid state spectral filters whose transmission characteristics can be controlled solely by the application of an electric field. Research into such devices was based on the premise that the electro-optic effect in certain crystals could be employed to replace the mechanical tuning sections of a birefringent interference filter. The initial motivation for tunable filter development came from several potential applications, ranging from color projection systems to astronomical telescopes, for which simultaneous spatial and spectral resolution was required. Subsequently, the need for remote sensing capabilities and, in particular, space-based platforms, has motivated the development of devices which are both compact and free of the difficulties inherent in mechanically tuned spectral filters. In many of these applications restrictions on size, weight, and power consumption have made the use of compact, solid state tunable filters highly desirable. Tunable filters are incorporated in these systems to enhance the probability of target detection by identification of spectral signatures and to eliminate undesirable optical signals from the sensor.
Optical filters can be spectrally tuned by the use of optical materials whose refractive indices are modified by the application of an electric field. Such materials include those which exhibit linear (Pockels) or quadratic (Kerr) electro-optic effects in which an index of refraction change results from a non-linear response of the crystalline material. In addition, liquid crystal materials, in which large index changes result from an electric field-induced rotation of optically anisotropic molecules, may be employed. Among the tunable filter designs available in the art are electro-optic birefringent interference filters, Fabry-Perot devices, and electro-optic coupled-wave filters. These filters can be employed as passband filters, in which a narrow band of wavelengths is transmitted through the filter and the remainder of the incident light beam is rejected, or as stopband filters, which pass all wavelengths except for a narrow band of light. In the passband configuration, such filters can be made highly efficient, so that nearly 100% of the light at the desired wavelength is transmitted through the filter. Where it is necessary to exclude a particular wavelength, however, these filters are not as successful, since the percentage of light excluded by such a filter at stopband wavelengths cannot readily be made close to zero. Consequently, a technique which would permit the highly efficient separation of a particular wavelength from a beam of light would be welcome in the optical filter art.